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2. 2 SPECTRAL REFLECTANCE MEASUREMENT METHODOLOGIES FOR TUZ GOLU FIELD CAMPAIGN Y. Boucher, F. Viallefont, A. Deadman, N. Fox, I. Behnert, D. Griffith, P. Harris, D. Helder, E. Knaeps, L. Leigh, Y. Li, H. Ozen, F. Ponzoni, S. Sterckx

3. 3 Outline  Introduction  Principles and instrumentations used for the field reflectance measurements  Measurement spatial sampling strategies  Results and conclusion

4. 4 Introduction Vicarious calibration = comparison of the radiance measured on-orbit and the radiance propagated from the optical or radiative properties of the measured reference test site  Radiance-based method: field upwelling radiance measurement at the time of the satellite acquisition + propagtion  Reflectance-based method: field reflectance measurement (same illumination conditions) + meas/model of the irradiance + propagation Advantages: more accurate and detailed site characterization One of the objective of 2010 CEOS Key comparison: Tuz Gölu campaign, was to compare different methods of field reflectance characterization for satellite calibration.

5. 5 Principle of field reflectance mesurement (1) Aim : Vicarious calibration of multispectral satellite sensors  Measurements of radiance and reflectance meas. have to be spectrally resolved  Ground measurements done with a spectroradiometer Meas. on the surface at t 1 Meas. on the reference at t 2 Measurement of the reflectance is relative to a reference: 1)Meas. of the radiance of the surface of interest at t 1 2)Meas. of the radiance the reference at t 2 R ground = R ref x S ground (t1) / S ref (t2)

6. 6 Possible compensation of irradiance temporal variation:  Measurement on the reference before and after measurement on the surface R ground = R ref x S ground (t 2 ) / S ref-mean with S ref-mean = [S ref (t 1 ) + S ref (t 3 ) ] / 2 Principle of field reflectance mesurement (2) Meas. on the surface at t 2 Meas. on the reference at t 1 Meas. on the reference at t 3

7. 7 Possible compensation of BRDF effects: -Usually, the reference panel reflectance factor (calibration) R ref = directional (8°)- hemispherical reflectance -The reflectance factor R surface of the ground and of the panel measured outdoors = bidirectional reflectance + hemispherical-directional reflectance (because of the direct + diffuse illumination conditions) -But the reference panel (like spectralon) is nearly lambertian but not exactly Principle of field reflectance mesurement (3) R D-H ref 8° IFO V R surfac e  BRDF correction on the reference panel reflectance R ref is suitable, according to the solar zenith angle and the viewing zenith angle, for applying the equation: R ground = R ref (  sun,  v ) x S ground / S ref

8. 8 Instrumentation for spectral reflectance measurements of Tuz Gölü site: Sensors  CMA/NSMC, CSIR, GISTDA, KARI, ONERA, SDSU, TUBITAK and VITO used an ASD Fielspec spectroradiometer, calibrated in radiance 8° FOV  10-20 cm (depending on h) h Spectral domain 0.35-1µm1-1.8µm1.8-2.5µm DetectorCCDInGaAs Spectral resolution 3nm8.5nm6.5nm Spectral sampling 1.4nm resamp 1nm 2nm resamp 1nm 2nm resamp 1nm Measure- ments Sample: 10 spectra averaged Dark current: 25 spectra averaged  INPE used a CIMEL 313 field multispectral radiometer 5504506508501500nm FOV: 10° 5 bands (filter wheel) Calibrated in radiance

9. 9 Instrumentation for spectral reflectance measurements of Tuz Gölü site: Reference panels  All the reference panels were spectralon ® from Labsphere, except one of NPL (Gigahertz Optik) and the one of CMA (Hefei Institute of Physical Science)  Reference panel were calibrated: Directional-hemispherical reflectance, by Labsphere (CSIR, KARI, Tübitak) Directional-hemispherical reflectance, by themthelves (NPL, Onera, INPR) BRDF (SDSU, CMA)

10. 10 Site description and spatial sampling of ground-based reflectance measurements AVNIR-2 acquisition image of Tuz Gölü site on 15 Aug 2010 Problem:  Area wide enough for satellite calibration  Small enough to be measured on the ground in a reasonable time  Which sampling strategy for ground-based reflectance measurement?

11. 11 Sampling strategies Challenges:  Giving a good assessment of the mean reflectance of a (large) area  Giving variations around this mean  Do all the measurements in less than one hour if possible 3 sampling strategies:  Spaced out sampling averaging several measurements over one point  Space out sampling with estimation of the local spatial variability  In-motion sampling etc 10

12. 12 Spaced out sampling averaging several measurements on each point (1)  Regular grid with spaced out sampling points  Several measurements are averaged for each sampling point  allows evaluation of the type A uncertainty (repeatability) Strategy used by Onera for 300m x 100m areas 21 sampling points. Time of measurement: 1 hour For each sampling point: Optimization, Ref.x5, ground meas.x10, ref.x5 40 m 20m 30m 20m 30m etc 10

13. 13 Spaced out sampling averaging several measurements on each point (2) A similar strategy had also been used by:  CSIR: for each point: optimisation, 5 references, 5 ground measurements CSIR sampling for M3 300m x 100m area

14. 14 Spaced out sampling averaging several measurements on each point (2) A similar strategy had also been used by:  CSIR: for each point optimisation, 5 references, 5 ground measurements  INPE: 1h30 for ~30 points, for each: 5 references, 5 ground measurements INPE sampling for one 300m x 100m area

15. 15 Spaced out sampling averaging several measurements on each point (2) A similar strategy had also been used by:  CSIR: for each point optimisation, 5 references, 5 ground measurements  INPE: 1h30 for 29-30 points, for each 5 references, 5 ground measurements  Tübitak Uzay: 15 to 30 sampling points, ref. every 10-15min, 10 meas. averaged for ref and ground Tübitak Uzay's sampling for one 300m x 100 m area

16. 16 Spaced out sampling averaging several measurements on each point (2) A similar strategy had also been used by:  CSIR: for each point optimisation, 5 references, 5 ground measurements  INPE: 1h30 for 29-30 points, for each 5 references, 5 ground measurements  Tübitak Uzay: 15 to 30 sampling points, ref. every 10-15min, 10 scans averaged for ref and ground  CMA: 11 sampling points in 1h30min, for each point: 2 ref. and 5 ground meas. CMA's sampling for the M9 1km x 1km area

17. 17 Spaced out sampling with local variability measurements As previous strategy, uses a regular grid with spaced out sampling points For each point of the grid, several meas. are locally recorded over different points near the nominal location etc Strategy used by VITO: for each sampling point: 5 ref., 4 points (4 meas. per point), 5 ref. VITO's sampling strategy for the 300 m x 100 m areas (25 points) Su n Local measurements for each sampling point VITO's sampling strategy for the M9 1km x 1km area (11 points) Drawback: time consuming

18. 18 In-motion sampling (1) The “In-motion” technique was developed by University of Arizona and South Dakota State University (SDSU). The ASD spectrometer operator is moving continuously and measuring every few seconds (  few meters). Ref. panel is measured at the beginning and the end of the transect. Advantage: high number of measurements are done in a short time Strategy used by SDSU: 20 meas. continuously acquired / 2.5m, averaged in one sample. 100 samples / row. 300m x 100m area measured in 30 minutes North Ref. panel meas. 8° FOV ~10cm x 2.5m

19. 19 In-motion sampling (2) A similar strategy had also been used by:  CSIR: for each transect: optimisation, 5 ref, continuous measurements, 5 ref at the end CSIR in-motion sampling for M3 300m x 100m area

20. 20 In-motion sampling (2) A similar strategy had also been used by:  CSIR: for each transect: optimisation, 5 ref, continuous measurements, 5 ref at the end  Onera for M9 1km x 1km area: 10 meas. continuously acquired / 15m-20m (10s), averaged in one sample. ~50 samples / transect. Onera in-motion sampling for M9 1km x 1km area

21. 21 Results comparison (1)  All teams pre-process the data, and supplied to NPL the reflectance factor R( ) with the associated standard uncertainty Type A and Type B  NPL re-calculated the RF of the site, as a weighted mean of the individual sampling points values and the associated standard uncertainty to this weighted RF value  The results are still preliminary, since some data of some teams have to be recalculated.  The plotted RF had been measured at different dates (17 to 25 August 2010) – for similar sun zenith angles  Good agreement (+/- 0.02) M4 300m x 300m area reported reflectance

22. 22 Results comparison (2)  Results on M9 1km x 1km site:  (Less) good agreement (+/- 0.05 in the visible, +/- 0.03 in the SWIR  Site bigger  higher sensitivity to spatial variability;  Longer time of measurement  different sun Zenith angles M9 1km x 1km area reported reflectance  Possible reasons of the differences:  Measurements bias and uncertainties,  Temporal variability of the reflectance (solar zenith angle difference 30° to 40°, soil humitity content)  Slight directional effects on ref. panel  Spatial variability

23. 23 Conclusion Ground-based reflectance measurements of Tuz Gölü site has been made by 9 teams  Similarities:  Instrumentation (ASD fieldspec, except INPE)  Preliminary intercalibration (spectroradiometers and reference panels)  "Best practise sharing" before the experiment (Tuz Gölü 2009)  Differences:  3 different sampling strategies: spaced out sampling, spaced out sampling with local variability, in motion sampling  Non simultaneity of the measurements  Promissing results:  Good agreement between the measurements made by the different teams  Some reprocessing in progress…